After about two decades of poking around Mars, it’s clear that scientists don’t expect to find life, certainly not on the surface [SciTechStory: Life on Mars: If it exists, is below the surface]. There are no Martian yetis, or if any life at all, nothing bigger than a bacteria – probably living deep below the surface. So the excitement about the announcement that the U.S. Mars rover Curiosity has identified a “life friendly environment” in a former lake bed is not about today’s conditions, but the conditions that might have supported life billions of years ago.

Keep in mind that even former “life” has yet to be discovered on Mars, either direct (as in a fossil) or indirect (chemical traces). For now, scientists – mostly the people of the nascent field of exobiology – utilize suppositions about life that we’ve gleaned from studying life on Earth. It may turn out that life, if it ever existed on Mars, may have had different signatures than we expect – but that’s the importance of what’s happening now with the new data provided by Curiosity.

The Curiosity rover is traversing the Gale Crater, a 150-kilometer (90 mile) wide impact basin with a mountain in the center. It’s looking at the geology as it goes, sampling the soils, digging a bit, sniffing the air and taking jillions of images. What it sees now, in a location named “Yellowknife Bay” (remembrance of a place in the Yukon of Canada) is an area that almost certainly was a lake roughly 3.6 billion years ago.

It’s interesting to note that the descriptive language NASA uses about Mars now takes the former existence of water for granted. It’s gone now, from the surface except in frozen form near the poles, but water signs are everywhere on Mars and most scientists now believe that water – the foundation of life as we know it on Earth – was once abundant on Mars.

The evidence of water in Yellowknife Bay is in the form of mudstone, a sedimentary material typically laid down on the bed of a relatively calm, shallow and freshwater lake. The analysts looking at Curiosity’s data suggest that the lake could have existed for a long as 100,000 years – long enough to develop life if the conditions were favorable. Analysis with x-ray diffraction techniques reveal that the mudstone (smectite) contains minerals crucial for life, including carbon, hydrogen, sulfur, nitrogen and phosphorus. The same materials show that the lake had a low salinity (not too much salt) and a neutral pH, neither too acidic nor alkaline to inhibit life.

On the other hand, most life on Earth used sunlight (directly or indirectly) as a source of energy. On Mars the sunlight is weaker, most scientists believe too weak to produce life (at least as we know it). If life existed on Mars, it had to derive its energy from chemical reaction, which makes the presence of so many key elements necessary for the right chemistry to happen.

Whether that chemistry ever happened and whether this or other Martian lakes existed long enough to produce life – that’s the target for Curiosity’s next round of investigation.